1. Field of the Invention
The present invention is an ultrasound probe positioning immersion shell that positions and aligns a diagnostic or therapeutic device, such as an ultrasound transducer probe, for the purpose of immersion of the eye for ultrasound axial length measurement, anterior chamber depth measurement, retinal detail measurement, or other diagnostic or therapeutic applications requiring alignment with the long axis of the eye.
2. Description of the Prior Art
There are several situations in the treatment of an ophthalmic patient that require a diagnostic ultrasound examination providing detailed information of the anatomical structures of the eye. This information enables the physician to provide the best possible care for a large variety of ocular disorders.
The most frequent use of ultrasound in ophthalmology is the axial eye length A-scan used to measure the eye prior to cataract surgery. A synonym for this type of A-scan is biometry. This measurement of an eye's axial length provides one of the three important values needed to calculate the appropriate power of an intraocular lens (IOL) implant after cataract removal. An inaccurate axial length measurement of just one millimeter can result in a post-operative optical error of three diopters, enough to necessitate a second surgery. Cataract removal and insertion of an intraocular lens (IOL) is performed over 1.5 million times a year in the U.S.
Two of the most commonly used techniques to perform axial eye length measurements are as follows:                a. Applanation, a contact technique; and,        b. Immersion, an ultrasound, non-contact technique.        
With the contact method, the axial length is measured with the ultrasound probe applanated on the center of the cornea. The biometrist must ensure that neither ointment nor excess fluid (e.g., anesthetic drops or tears) are present on the cornea prior to beginning the examination, since even a small amount of fluid may lead to erroneous axial length readings. The contact technique can be performed by applanation (chin rest method) or by hand (hand-held method). Disadvantages of the contact technique include both corneal compression and the possibility of corneal abrasion. The anterior chamber depth must be evaluated in each echogram since shallowing of the chamber occurs when the cornea is indented. Further, due to examiner parallax or alignment problems, it is often difficult to be sure measurements are taken from the center of the cornea.
The immersion non-contact biometry method is the preferred method of accurately measuring the length of the eye using a special shell that provides a liquid bath between the front surface of the eye (cornea) and the measuring ultrasound probe. The unique feature about the immersion method is that the ultrasound probe never actually touches the eye. This has value since one of the most common errors made while performing an applanation (contact) axial eye length A-scan is the compression or flattening of the cornea, producing a falsely short measurement. This technique is also best for patients with blepharospasm and fixation difficulties.
The three fundamental advantages of the immersion method are the following:                a. the capability to prevent inaccuracies due to corneal compression by eliminating the need to touch the cornea;        b. the capability to reproduce the measurement more readily; and,        c. the capability to use echoes from the cornea for aligning the sound beam along the visual axis, thereby providing additional assurance of a measurement to the macula.        
With the advancement in IOL design and manufacturing, a more precise axial length measurement of the eye is required for determining the correct IOL power required for optimal pseudophakic correction. An inaccurate axial length measurement of only 1 mm can result in a significant post-operative refractive error. The ultrasound manufacturers have improved the A-scan equipment with upgraded hardware and software for measuring eye length with the transducer immersed in a liquid medium. The biometry instrument converts the time readings into millimeter axial length. When using the immersion technique, these A-scan improvements require that the ultrasound probe tip be placed at a fixed and specified distance from the corneal surface. For accurate measurements, it is essential that the ultrasound probe remains perpendicular to the visual axis while the transducer is submersed. It is equally important for the liquid medium between the corneal surface and the transducer to be free of trapped air bubbles. The presence of air bubbles can disrupt the sound wave transmission and interfere with axial length measurement.
To keep the eye submersed in a liquid medium during biometry, various cylindrical shells of different shapes are used in immersion A-scan. All have shortcomings.
Hansen Shell
The Hansen shell is simply a plastic cylinder open at both ends incorporated in a two-handed procedure requiring skill to master. The Hansen shell is inserted under the eyelids and hand-held while the liquid medium is poured from the top submerging the transducer and eye. Because the ultrasound probe is free to move, it can be easily moved vertically and tilted, resulting in erroneous measurements. Further, a viscous solution, Goniosol, is required, which is expensive and leaves a vision-blurring film. Achieving accurate measurements using this shell design is difficult to master.
Kohn Shell
The Kohn shell has an hourglass shape with the ultrasound probe inserted to the constriction. A port including a metal tube and hose is located at the bottom portion or lower chamber of the shell for introducing the liquid medium. The ultrasound probe and shell meet at one location with a larger diameter opening at the top of the shell. This can result in vertical and angled error due to a single fulcrum contact point, as with mating two cones with different dimensions and angles. Any dimensional difference between the ultrasound probe shape and the shell constriction increases the likelihood for the ultrasound probe to be tilted and/or positioned at a different height.
This Kohn shell design forms two chambers once the ultrasound probe is inserted, and a “cork effect” occurs at the contact point between the ultrasound probe and shell constriction. The liquid medium then must be injected after the shell is on the eye through the port located in the lower chamber between the constriction and the bottom of the shell contacting the eye. This reduces the ability to visually place the shell and ultrasound probe on the eye due to the port and any connected tubing blocking the view. Furthermore, due to the ultrasound probe blocking the air from escaping from the lower chamber, a large air bubble can be easily trapped in the lower chamber and prevent an ultrasound axial length measurement.
Prager Scleral Shell
Another current design is the Prager scleral shell. This is a polycarbonate plastic cylinder with a flanged end that contacts the eye. To accommodate the ultrasound probe, the upper portion of the shell is bored out in the center with an inner diameter slightly larger than the ultrasound probe to maintain orientation. A setscrew located at the top of the shell is then tightened against the probe to preclude the probe from protruding out the shell bottom and potentially contacting the cornea. The probe tip can be placed at any height from the cornea. This requires the operator to carefully inspect the ultrasound probe height position before every ultrasound exam. If the ultrasound probe tip is either too low or too high, a faulty reading will occur. Furthermore, the ultrasound probe can be easily canted from the perpendicular position in the shell when the setscrew is tightened. To fill the shell with the liquid medium, a metal port or filler tube is press fit into the shell wall. The metal port or filler tube is inserted into PVC tubing or a butterfly needle is inserted into the metal port or filler tube. Any sharp object in close proximity to the eye is considered a safety issue. Typically, multiple holes are drilled into the wall of the lower portion of the shell for air to escape as the liquid enters the lower portion of the shell.